I love any epic musical moment; I'm pretty into the film score; Howard Shore is THE MAN. I love:
RotK: Aragorn's speech at the march to the Black Gate.
TTT: Too many! Firstly, when Aragorn barges into the
Great Hall all bloody and tattered after everyone thinks he's dead. Secondly, the battle of Helm's Deep, when Aragorn calls on the elves to fire, in elvish, of course. (movie)
FotR: When the Fellowship first sets out, right after the Council convenes at Rivendell; the movie really sets a great cinematographic experience, and a main LotR theme really comes forth in the scene. Love it!
And lastly, the opening sequence of the Battle of
the Last Alliance of Elves and Men was pretty epic, with Elrond in full armor, gear, and shooting commands as if he had a quiver full of them. Epic!
(I'm referring to the movies for all these, of course)